1. Field of the Invention
The present invention relates to a solid electrolytic capacitor used for various electronic equipment, and a method of manufacturing the same.
2. Background Art
With the recent development of digital equipment, there is a strong demand for a capacitor that is lower in equivalent series resistance (hereinafter referred as ESR) and excellent in high frequency characteristics. In order to meet such a demand, those using a solid electrolyte layer of manganese dioxide, polypyrrole, or polythiophene as electrolyte have been developed and commercialized.
FIG. 2 is a sectional view of a conventional solid electrolytic capacitor. The solid electrolytic capacitor includes capacitor element 5, anode terminal 6, cathode terminal 8, and exterior resin 9. Capacitor element 5 is formed of anode body 1, dielectric oxide layer 2, solid electrolyte layer 3, and cathode layer 4.
Anode body 1 is formed by sintering valve metal such as aluminum and tantalum as a porous body. Anode lead-out portion 1A is disposed so as to protrude from anode body 1. Dielectric oxide layer 2 is formed on the surface of anode body 1 by using an anode oxidizing method. Solid electrolyte layer 3 contains a conductive polymer such as polypyrrole and is formed on the surface of dielectric oxide layer 2. Cathode layer 4 is formed of carbon layer 4A and conductor layer 4B made from silver paste, and formed on the surface of solid electrolyte layer 3. Anode terminal 6 is connected to anode lead-out portion 1A, and cathode terminal 8 is connected to conductor layer 4B via conductive adhesive 7. Anode terminal 6 and cathode terminal 8 respectively include connections 6A and 8A to be connected to an electronic circuit. Insulating exterior resin 9 covers capacitor element 5, anode terminal 6, and cathode terminal 8 except connections 6A and 8A.
In a solid electrolytic capacitor having a configuration as described above, solid electrolyte layer 3 is very low in specific resistance. Accordingly, the solid electrolytic capacitor is low in ESR. Such a solid electrolytic capacitor is for example disclosed in Unexamined Japanese Patent Publication No. 2001-284182.
However, in the case of the conventional solid electrolytic capacitor, when exposed in a high temperature environment, ESR tends to increase with time. The following can be considered as a main reason of such a phenomenon.
In a high temperature environment, solid electrolyte layer 3 containing a conductive polymer tends to peel itself off from carbon layer 4A formed thereon. Accordingly, the interfacial resistance increases between solid electrolyte layer 3 and carbon layer 4A. Also, external oxygen or water entering from gaps created due to peeling solid electrolyte layer 3 off from carbon layer 4A increases the specific resistance of solid electrolyte layer 3, itself. As a result, ESR of the solid electrolytic capacitor is liable to increase with the lapse of time.